1. Field of Invention
The present invention relates to a light emitting device driver circuit and a control circuit and a control method thereof. Particularly, it relates to such light emitting device driver circuit and control circuit and control method thereof which improve a dimmable range of the light emitting device circuit.
2. Description of Related Art
FIG. 1A shows a schematic diagram of a prior art light emitting diode (LED) power supply circuit 10. As shown in FIG. 1A, the LED power supply circuit 10 includes a tri-electrode AC switch (TRIAC) dimming circuit 11, a rectifier circuit 13, an LED driver circuit 15, and a bleeder circuit 17. The TRIAC dimming circuit 11 receives an AC signal AC and outputs an AC dimming signal AC1′. When the AC signal exceeds a trigger phase, the TRIAC dimming circuit 11 fires (starts-up) and is turned ON. FIG. 1B shows a schematic diagram of waveforms of an AC voltage ACV and an AC dimming voltage ACV1′, wherein the AC signal includes the AC voltage ACV, and the AC dimming signal AC1′ includes the AC dimming voltage ACV1′. The AC voltage ACV is shown by a dashed line, and the AC dimming voltage ACV1′ generated by the TRIAC dimming circuit 11 is shown by a solid line. The TRIAC dimming circuit 11 is triggered at the trigger phase, and is turned ON for an ON phase period ONPP, whereas the period in which the TRIAC dimming circuit 11 is turned OFF is an OFF phase period OFFPP. The rectifier circuit 13 receives the AC dimming signal AC1′, and rectifies it to generate a rectified dimming signal, which includes a rectified dimming current Ir1′. The rectified dimming current Ir1′ is divided to an input current Iin1 flowing through the LED driver circuit 15, and a bleeding current Ibld flowing through the bleeder circuit 17. The LED driver circuit 15 converts a rectified dimming voltage Vin to an output voltage Vout which is provided to the LED circuit 20. In the aforementioned circuit, the TRIAC dimming circuit 11 functions to determine the trigger phase of the AC dimming voltage ACV1′, so as to adjust an average brightness of the LED circuit 20. The LED driver circuit 15 includes a power stage circuit which has at least one power switch. The power stage circuit may be a synchronous or asynchronous buck, boost, inverting, buck-boost, inverting-boost, or flyback power stage circuit as shown in FIGS. 6A-6K.
One of the drawbacks of the aforementioned prior art is that the TRIAC dimming circuit 11 includes a TRIAC device which requires a large latching current to fire (start-up), and after the LED circuit 20 is turned ON in the ON phase period ONPP, it is required for the current flowing through the TRIAC device to have an absolute level higher than a holding current in order to keep the TRIAC dimming circuit 11 in a normal operation. If what the power supply drives is a high power consuming load circuit, such as a conventional incandescent lamp, the latching current and the holding current for the TRIAC device is sufficient. However if what the power supply drives is a low power consuming load circuit, such as the LED circuit 20, the latching current and the holding current for the TRIAC device is insufficient because of the low current of the LED circuit 20. If the power supply circuit does not generate a sufficient latching current to fire the TRIAC device or the holding current to keep it operating normally, a so-called “misfire” occurs and the LED circuit 20 will flicker perceptibly, or a dimming range of the LED circuit 20 is limited. In this regard, FIG. 1C shows the waveforms of the AC voltage ACV and the AC dimming voltage ACV1′ when the misfire condition occurs. On the other hand, even though the latching current is sufficient to fire the TRIAC device, the misfire may still occurs or the dimming range of the LED circuit 20 is limited if the AC current of the AC signal flowing through the TRIAC device is too low, for example when the trigger phase is too high (late) such that the absolute level of the AC current flowing through the TRIAC dimming circuit 11 is lower than the holding current.
FIGS. 1D and 1E show schematic diagrams of signal waveforms of an input current Iin1, the bleeding current Ibld, and the rectified dimming current Ir1′, which illustrates how the bleeder circuit 17 maintains the absolute level of the AC current flowing through the TRIAC dimming circuit 11 to be higher than the holding current. The bleeder circuit 17 is coupled between the rectifier circuit 13 and the LED driver circuit 15, for generating a sufficient latching current periodically to trigger the TRIAC device in the TRIAC dimmer circuit 11, and maintaining the absolute level of the AC current flowing through the TRIAC dimming circuit 11 to be higher than the holding current. The bleeding current Ibld must not be lower than the holding current, such that the TRIAC dimming circuit 11 can operate normally.
Although the prior art shown in FIG. 1A mitigates the problem of flickering and improves the dimming range, it still has several drawbacks. First, the TRIAC dimming circuit 12 can not perform full range dimming; the trigger phase can not be higher (later) than a specific phase because of the bleeder circuit 17. Second, the bleeding current generated by the bleeder circuit 17 is consumed unproductively; it is wasted because it does not flow through the LED circuit 20, and it may also cause an overheat problem. Third the prior art requires a large circuitry size because of the bleeder circuit 17.
In view of the foregoing, the present invention provides a light emitting device driver circuit and a control circuit and a control method thereof. In the present invention, the absolute level of the AC dimming current is maintained not lower than the holding current by the control circuit which generates the operation signal to operate at least one power switch of the power stage circuit.